1. Field of the Invention
The present invention relates to the field of cursor control devices for video displays, commonly referred to as a "mouse", and more particularly to a rotatable ball used in the mouse.
2. Prior Art
In many video display systems for computers it is desirable to control the positioning of a cursor on a video display screen by means other than a keyboard. Joy-stick controllers and rotating, tracking balls are just two examples of devices for controlling the positioning of a cursor on the display screen. Another commonly used cursor controlling device is referred to as a "mouse".
In a common embodiment of a mouse, a hand-held housing encases a substantially spherical ball which is free to rotate within the housing. The ball is also free to "float" in the vertical direction within the housing, but remains in contact with a working surface during use. When the mouse is placed onto the working surface, which is typically a friction pad, the ball makes contact with this working surface and rotates as an operator manually moves the mouse over the working surface. The mechanical movement of the mouse causes the ball to rotate within the housing of the mouse, wherein the direction of ball movement is relative to the mouse movement. The ball movement is then translated to electrical signals for controlling the position of the cursor.
In a typical mouse, roller shafts are located within the housing and are in contact with the ball. The rotation of the ball causes one or more roller shafts to rotate. The roller shafts are mechanically coupled to a photo-emitter and a photo-detector to convert the mechanical motion of the roller shaft to electrical signals. The output from the photo-detector provides the necessary translation for providing the cursor movement on the display screen corresponding to the mechanical movement of the mouse. One such example of a mouse is disclosed in U.S. Pat. No. 4,464,652.
Although improvements have been made to the mouse, the improvements were typically centered on the electrical-mechanical aspects of the mouse. However, a major disadvantage resides in the balls utilized with most mice. One such disadvantage of prior art mouse balls relates to the location of the center of gravity of a given ball. That is, the ball's center of gravity is at a point other than the actual geometric center of the substantially spherical ball. Whenever the ball's center of gravity is not at its geometric center, the ball will have a tendency of not rolling uniformly.
To illustrate, it is useful to visualize the ball rotated in place about its geometric center and above a reference plane created by the working surface. As the ball is rotated, the gravitational center point of the ball is located at different points in relation to the reference plane. The gravitational force between the working surface and the ball will either pull the ball towards the surface in the direction of the rotational movement or pull the ball in the opposite direction of the rotational movement. Depending on the direction of rotation and the location of the gravitational center in reference to the geometric center, the gravitational force will either increase or decrease the rotational velocity of the ball as it is rotated.
Therefore, when a user directs the mouse to move along the working surface, the ball's rotational velocity will vary periodically due to the above described defect and without regard to the speed at which the user moves the mouse. This causes the ball to "jump" along the working surface, which in turn causes the roller shafts to respond accordingly. This "jumping" or "jitter" effect causes the cursor to respond with a dis-continuous movement on the screen. As a result, an operator moving the mouse at a constant velocity will see the cursor moving at a varying velocity on the screen, instead of the desired constant velocity movement of the cursor. The amount of the variance of the velocity will dependent on the distance differential between the geometric center and the gravitational center of the ball. The operator will notice the cursor to "jitter" as it is moved across the display screen.
The above described problem, which is associated with some of the prior art balls, results because these balls are comprised of an inner core and an outer core (outer covering) surrounding the inner core. An inner core is necessitated by a requirement of having minimum weight associated with these mouse balls. Because the ball is free to "float" within the housing, a certain specified minimum weight is required for it to maintain frictional contact with the working surface. Below a minimum specified weight, the balls will "float" upwards from the working surface and will not rotate as the mouse is moved across the working surface.
The inner core is comprised of a material which provides the necessary weight. The outer core is comprised of a lighter weight material which is more suited for providing the surface which makes contact with the working surface. Also, it is essential that this outer surface be non-conductive, so that the ball does not become an electrical conductor within the mouse or between an electrical part of the mouse and the working surface.
During construction of these type of prior art balls, having an inner core and an outer covering, it is difficult to match the geometric and gravitational centers of the inner core and the outer covering. This problem associated with the coincidence of the center of gravity to the geometric center of the above described prior art ball has necessitated the testing of the ball to determine if the gravitational center is so removed from the geometric center as to cause the earlier described "jumping" phenomena to be noticeable. To make such a determination, each ball must be tested individually, such as by watching the ball's rotational action across a flat surface or actually using the ball within a mouse and watching the cursor response on a display screen. Such testing techniques are costly and time consuming. In addition, balls which do not meet the requisite criteria are deemed unacceptable and discarded. This discarding of unacceptable balls impacts on manufacturing production yield, leading to added manufacturing costs.
As a result, a uniform composition ball not having an inner core is desired. However, such a ball, which has uniform composition, must still meet the minimum weight requirement for use in a given prior art mouse. The ball must also have a non-conductive surface, so that it does not cause the mouse to electrically "short out". Conventional black rubber balls could possibly be adapted to meet this requirement, assuming that the density of the rubber is such that it meets the minimum weight requirement, however, conventional black rubber has a distinct disadvantage in that it tends to leave black marks on the working surface. Further it has the characteristically unpleasant odor associated with rubber.
Accordingly, it is appreciated that what is needed is a uniform composition, non-conducting mouse ball made from a material or materials other than black rubber, and has sufficient density to function as a mouse ball.